Method and apparatus for continuous monitoring of road surface friction

ABSTRACT

A method and apparatus for continuously or repeatedly monitoring road surface friction utilizes a separate test wheel attached to a vehicle and in contact with a road surface and a device for detecting slippage of the test wheel resulting from applying either a braking or accelerating torque on the test wheel. A signal corresponding to the applied braking or accelerating torque at the moment slip is detected and used to provide an indication to the driver of the slip condition. The method and apparatus is useful in vehicles as well as aircraft or in other applications where it is desired to monitor road friction. An embodiment of the invention applies and maintains a measured vertical force to the surface of the test wheel of the friction monitor utilizing an electromagnetic force field under processor control. The processor can combine signals from the vertical force torque motor circuit, and the test wheel torque motor circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation in part of application Ser.No. 08/843,960 filed Apr. 17, 1997; which is a continuation of Ser. No.08/403,106, filed Mar. 13, 1995, now abandoned.

FIELD OF THE INVENTION

[0002] The invention relates to monitoring a road surface condition onwhich a vehicle is travelling, and more particularly to a method andapparatus for continuously determining a slip condition of a vehiclewhich is directly related to the coefficient of friction of the roadsurface.

[0003] The invention also relates to a method and apparatus for applyinga controlled vertical (i.e., normal) force to a test wheel of a frictionmonitor and to a friction monitor using such a vertical force monitor incombination with an electromagnetic torque motor for producing slippageof a test wheel.

[0004] The invention further relates to a device for monitoring asurface (e.g., road, runway, rail line etc.) condition on which amonitor (e.g., vehicle, airplane, train etc.) is travelling, and moreparticularly to a method and apparatus for determining a slip conditionof a vehicle which is directly related to the coefficient of friction ofthe road surface while utilizing an electromagnetically controlledvertical force applied to the test wheel.

BACKGROUND OF THE INVENTION

[0005] The availability of quantitative information representative ofthe coeficient of friction of the road surface is very beneficial todrivers of moving vehicles, including planes. An exact knowledge of howslippery the surface is, continuously provided, gives the driversignificant advantages in determining safe speeds, distances from otherautomobiles, acceleration and braking patterns etc. so as to permit asafer operation of the vehicle, plane or apparatus being driven.

[0006] Previous devices for monitoring the coefficient of friction hadsignificant disadvantages such as, for example: a failure to providecontinuous road friction data over long distances; the use of complexelectro-mechanical-hydraulic mechanisms with unfavorable wearcharacteristics; limited, specificity, variability and responsiveness oftest wheel(s) braking force; the use of only indirectly measured valuesof the force (torque) required to produce slippage; employment ofequipment which was unsuited for continuous use close to the ground andin inclement conditions; the use of a cumbersome test wheel suspensionand carriage; employment of equipment which produced adverse affect onnormal drive operation; and the use of relatively complex designs whichproduces an expensive apparatus of only limited use.

[0007] When measuring the coefficient of friction, various means ofapplying vertical force have been employed, such as: springs, coils,rubber, compressed air bags, pistons containing air, gases, oil. Theforegoing vertical force applying devices have significant limitations.The vertical force tends to increase as tensioning means are compressedand decrease when decompressed. The vertical force is difficult tomeasure, especially if continuous or frequent measurements aredesirable. Further, one is generally not able to standardize the forcefrom one system to another. The vertical force tends to change withuse/wear due to deterioration of compressed materials/mechanisms, and tovary with change in temperature and/or moisture.

REVIEW OF PRIOR ART

[0008] Several patents are directed to devices which do not employ aseparate test wheel but rather employ one of the usual drive or drivenwheels of the vehicle. Among these patents are U.S. Pat. No. 4,882,693to Yopp and U.S. Pat. No. 4,545,240 to Leiber. These devices necessarilyadversely effect the driving characteristics of the vehicle.

[0009] The Yopp patent measures forces acting on a steering column, suchas the steering angle combined with other data.

[0010] Other patents teach the use of a test wheel or probe which islowered into contact with the road surface to perform the desiredmeasurement. Among these patents are U.S. Pat. Nos. 4,098,111 and4,212,063 to Hardmark; U.S. Pat. No. 4,958,512 to Johnson; U.S. Pat. No.3,893,330 to Shute; U.S. Pat. No. 4,315,426 to Brandon; U.S. Pat. No.4,662,211 to Strong; and U.S. Pat. No. 4,909,073 to Takahashi. Thesepatents employ complex devices which are not adapted for continuousoperation and wear and which are bulky and complicated in theiroperation.

OBJECTS OF THE INVENTION

[0011] One object of an aspect of the invention is to continuouslymonitor road surface friction characteristics under a wide variety ofroad conditions and vehicle velocities.

[0012] Another object of the invention is to provide a slippageindication by simulating the actual physical circumstances of slippageof a braked or accelerated wheel of a vehicle.

[0013] A further object of the invention is to monitor road surfacefriction without interfering with the movement of the vehicle wheels orbraking system such as would affect the directional path or speed of thevehicle, and in particular, not to affect the freedom of the drivewheels or other wheels of the vehicle or interfere with the brakingsystem, thereby potentially throwing the car off its directional path orcausing acceleration or deceleration of the vehicle.

[0014] Yet another object of an aspect of the invention is to provide alightweight, comparatively small monitoring device of suitable weightand size for mounting on an automobile or plane that operatesindependently of any other system in a moving vehicle, that iscomparatively inexpensive to manufacture, and that does not require muchmaintenance.

[0015] Another object of an aspect of the invention is to provide asimple, easily retractable, and easily detachable slip detection devicewhich does not constitute a significant obstacle to the movement of thevehicle.

[0016] Another object of an aspect of the invention is to continuouslyprovide near instantaneous, widely variable, highly specific, easilydirectly measurable, braking or accelerative force; which does not vary,fade, or fail, at any vehicular speed.

[0017] Another object of an aspect of the invention is to measurefriction conditions without utilizing cumbersome direct mechanicalfriction devices with their attendant mechanical wear.

[0018] Yet another object is to provide a slip detection device suitablefor after market mounting on existing vehicles.

[0019] It is a further object of an aspect of the invention to provide acompact suspension mechanism for applying and maintaining a controlled,accurately measurable vertical force to the test wheel of a frictionmonitor, which vertical force is unaffected by variations intemperature, moisture, or atmospheric pressure; remains constant as thetest wheel moves up and down with reference to its point of support dueto variations in the contour of the measured surfaces; and counters thetendency of the test wheel to move upwards and lose contact with themeasured surface. The vertical force may be varied in a controlledmanner during use.

SUMMARY OF THE INVENTION

[0020] The emphasis with the present invention, in accordance with anaspect thereof, is on the continuous determination of road surfacefriction characteristics for automobile drivers or pilots, so as tofacilitate decision-making with regard to safe travelling speed, brakingdistance, cornering speed, acceleration speed and the like in variousroad conditions; wet (hydroplaning), snow, slush, ice, oily surface,etc. Data on varying road coefficient of friction characteristics iscontinuously presented to the driver as a visual and/or auditory signal.

[0021] An embodiment of the present invention provides a means forcontinuously evaluating the coefficient of friction of the road byproviding a relative quantification of the coefficient of friction. Thedevice is comparatively compact and relays to the driver a visual and/orauditory signal indicating relative slipperiness of the road. Forexample, a suitably placed, non-distracting flashing light could beused; the flashing light can be designed to flash more frequently andmore brightly as the slipperiness of the road increases. An audio signalcan also be used to inform the driver about road conditions: again, thefrequency and intensity of the audio signal can be increased in relationto the slipperiness of the road.

[0022] An embodiment of the present invention mimics the strategycommonly used by drivers to evaluate road conditions: pressing on thebrake to determine how hard they have to press (decelerative force) inorder to produce a limited skid, alternatively, accelerating quickly, toobserve how much accelerative force is required to make the drive wheelsskid. This device repeatedly and automatically carries out thisfunction. Drivers know that the harder they have to press on theirbrakes or accelerator in order to produce wheel slippage, the lessskiddy the road is. This concept is the basis of the presentinvention—the more braking resistance or accelerative force required toproduce skidding, the greater the coefficient of friction.

[0023] The device, through the use of variable resistance oraccelerative force, creates slippage of a small test wheel and thendetects the earliest sign of skidding of this wheel and measures theamount of force required to produce slippage of the test wheel. Inaccordance with another aspect of the invention, a processor can combinesignals from the circuit regulating the vertical force torque device,with signals from any other circuits, in particular, the circuit tocontrol a torque motor used to produce slippage when measuring a surface(e.g., road, runway, rail line) condition.

[0024] An embodiment of the invention utilizes an electromagnetic forcefield, as distinct from direct mechanical means (springs, air pistons,rubber . . . ) to provide the requisite vertical force acting on thesurface of the test wheel.

[0025] Information regarding this force is then relayed to the driver.An elementary embodiment of the invention has an open loop controlsystem and a fixed electromagnetic field resistance to the rotation of atest wheel. A warning signal is provided to the driver when the testwheel skids indicating that the road surface has reached a potentiallyhazardous threshold.

[0026] Another embodiment of the invention utilizes an open-loop controlsystem and a variable braking resistance. A series of predeterminedresistance levels are applied, and the test wheel slip condition isconveyed to the driver at each state, providing a more refinedindication of road slipperiness.

[0027] Another embodiment of the invention utilizes a closed feedbackloop control system with variable braking resistance or accelerativeforce. The test wheel is first allowed to rotate freely as the vehicletravels. A timer initiates the test process, and variable resistance (oraccelerative force) is applied to the test wheel using, for example, anelectric motor, designed to produce torque in the same or oppositedirection (or same direction in the case of acceleration) to therotation of the test wheel. The braking resistance or accelerative forceis increased until the test wheel slips. A sensor determines therotational speed of the test wheel and, from this test wheel rotationalspeed in any given period of time, the amount of slip of the test wheelis determined. The amount of braking resistance or accelerative forcerequired to generate wheel slip is measured, and this information isconveyed to the driver by various display methods. The variable brakingor accelerative force is then decreased and the process is repeated.

[0028] Another embodiment of the invention employs a second or referencewheel. This reference wheel has no brake and serves solely as areference tachometer to facilitate determination of the percentage slipof the braked or accelerated test wheel. Optionally this reference wheelcan power a generator, which, generator can provide E.M.F. to theelectric motor and/or battery.

[0029] A test wheel is used, as opposed to actual vehicle wheels, toensure that the repeated road surface monitoring does not affect normalvehicle operation. The test wheel is designed to be easily retractableand instantly detachable to minimize the effects of roadway obstacles.The desire for simplicity and low cost leads to a method that usesincreasing braking resistance or accelerative force to induce slipinstead of a geared test wheel driven at a predetermined slip, and thatmonitors road surface conditions to produce a relative quantification ofthe actual coefficient of friction. Similarly, variable resistance torotation of the test wheel is provided by an electric motor, instead ofcumbersome mechanical brakes.

[0030] Another embodiment of the invention is directed to a method ofand apparatus for applying and maintaining a vertical force, which maybe variable if desired, to a test wheel of a friction monitor utilizingan electric torque device, such as a motor, under control of anelectronic processor such as a digital computer or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a block diagram of the road surface friction monitor;

[0032]FIG. 2 is a block diagram of an elementary embodiment of theinvention;

[0033]FIG. 3 illustrates the electric motor and rotation sensoraccording to the invention;

[0034]FIG. 4 illustrates a side view of the friction monitor;

[0035]FIGS. 5A, 5B, and 5C illustrate the folding mechanism of thefriction monitor;

[0036]FIG. 6 shows a plan view of the friction monitor;

[0037]FIG. 7 shows an alternate configuration of the friction monitor;

[0038]FIGS. 8A and 8B illustrate yet another embodiment of the inventionfor connecting an electric motor to a test wheel of the invention;

[0039]FIG. 9 is a block diagram of electromagnetic vertical (normal)force torque motor in combination with the friction monitor;

[0040]FIG. 10 illustrates the suspension mechanism for the test wheel,comprising a pivot with an electric torque motor adjacent to the testwheel; and

[0041]FIG. 11 illustrates in greater detail the pivot with the electricmotor adjacent to the test wheel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] The present invention is concerned with the determination of arelative variation in the coefficient of friction within a rangeaffecting safe operation of a moving vehicle by use of an arbitrarystandard. This arbitrary standard refers to the pre-determined constantdownward pressure (vertical force) on the test wheel, exerted throughthe suspension connecting member between the monitoring device itselfand the vehicle to which it is attached, the standard width and contourof the test wheel, and the standardized coefficient of friction of thetest wheel surface. Such a relative quantification provides formeaningful quantitative differentiation of the coefficient of frictionon the road surface. Vertical force sensor (154) where indicated,continuously monitors the vertical force, and signals from this monitorare continuously provided to the central processor.

[0043]FIG. 1 shows an overall block diagram of the friction monitoringsystem. The system comprises a test wheel 101, electric motor 103, testwheel rotation speed sensor 105, power supply electronics 107,controller 109, and output 113. The monitoring device may includevertical force sensor 154 for measuring the vertical force on the testwheel and feeding a signal indicative thereof to the controller. Thecontroller 109 includes a central processor 121 (i.e., a microprocessor)and a timer 123. The test wheel 101 is in contact with the ground andaccordingly rotates in the same direction as the wheels of the vehicle.The timer 123 (which may be a software timer configured within thecentral processor 121) periodically/repeatedly initiates the testsequence. The time interval between test sequences is variable, and istypically milliseconds to tenths of seconds.

[0044] Energy is supplied from the power supply electronics 107, underthe periodic direction of the timer 123 and controller 109, to theelectric motor 103. The electric motor 103 is designed to produce torquein the opposite direction from which the test wheel 101 rotates,creating a braking resistance on the test wheel 101, or the samedirection producing an accelerative thrust. The central processor 121increases the EMF to the electric motor 103 via the power supplyelectronics 107, creating a corresponding increase in the brakingresistance or accelerative force until the central processor 121 detectsslippage of the test wheel 101, measures, then reduces the EMF providedto the electric motor 103 via power supply electronics 107, decreasingthe braking resistance or accelerative force, and the test wheel 101 isfree to rotate in conjunction with the surface of the road until thenext test cycle initiated by the timer 123. The rotational speed of thetest wheel 101 is measured by the test wheel rotational speed sensor 105and a test wheel speed signal is conveyed to the central processor 121.Signals from vertical force sensor (154) are also conveyed to thecentral processor.

[0045] Optionally, a second free-wheeling wheel 111 which has the samediameter as the test wheel and which is not impeded by an appliedresistance is used as a reference/comparison tachometer. This referencewheel tachometer is free to rotate as the vehicle moves, the onlyresistance being created by its own bearings. A reference wheelrotational speed sensor 112 measures the speed of the reference wheel111, and supplies a reference speed signal to the central processor 121.Alternatively, a signal from the vehicle speedometer may be used toprovide the reference signal.

[0046] Where a reference wheel is deployed, the rotational speeddifferential between the test wheel 101 and the reference wheel 111provides a more precise indication of the extent of slippage of the testwheel 101, which can be expressed as a percentage, greater or lesser,than the reference wheel and provides the basis for the centralprocessor 121 to detect whether the test wheel 101 has slipped.

[0047] The central processor 121 is programmed to determine that a slipcondition (a first instance of a slip condition i.e. slippage up to onehundred percent, greater or lesser, than a reference wheel) isoccurring, its percentage slip, greater or lesser than the referencewheel, and continuously measures the EMF required to produce any givenpercentage slip. Slip curves graphically demonstrate coordinates ofbraking torque (vertical axis) and percentage slip (horizontal axis);maximum torque in relation to percentage slip, usually in the 10 to 20%range.

[0048] The torque, braking or accelerative, required to cause slippageof the test wheel will vary according to the degree of slip of the testwheel. Degree of slip can be expressed as a percentage increase ordecrease, in test wheel rotational speed, compared to a referencetachometer wheel. A braked, totally locked, non-rotating wheelcorresponds to one hundred percent slip. For purposes of computingrepresentative data on the coefficient of friction, it is valuable todetermine the relationship between the percentage slip and the torquerequired to produce it. If the test wheel 101 is rotating through snow,the snow tends to slow down the test wheel 101, and this “rollresistance” is reflected in the amount of energy required to accelerateor brake the test wheel 101. According to the invention, varying degreesof slip, from braking or accelerating force, can be created andcorrelated with the electric power (voltage supplied to the motor 103)required to create such varying degrees of slip, taking into accountroll resistance and other physical factors that affect braking. Brakingtorque is proportional to voltage supplied to the motor 103. Tabulationsof the braking torque in known varying slip conditions, as a function ofany degree of slip, at any given test wheel speed, may be stored in amemory of the central processor 121, which data can be combined with EMFtorque detection signal, and/or other signals, for further refinement offriction characteristics. Accordingly, the central processor 121 isprogrammed to measure the voltage going to the electric motor 103 at anygiven percentage slip (coordinate on the slip curve) up to a maximumdesignated percentage slip, then after measuring this voltage todecrease the voltage to the electric motor 103, decreasing braking oraccelerative forces acting on test wheel 101 and initiating another testcycle. Processor 121 can also be programmed to maintain percentage slipof the test wheel 101, so as to maximize the data available on thecoefficient of friction of the road. In general, the central processor121 may be programmed to use any electrical property of the torque motorto calculate the slip condition such as voltage, phase, current, poweror the like.

[0049] The measure of EMF at which slip of the test wheel 101 occurredis transferred from the central processor 121 via an EMF torquedetection signal to output block 113. The measured EMF is related to theslipperiness of the road, and the output block 113 conveys this roadsurface condition information to the driver in any number of methods,for instance as a flashing warning light 115 or audio warning signal 118previously described herein, or as a bar graph 116 or numeric display117. The skilled artisan will recognize that such information could bedelivered to the driver instantly and/or as an averaged value, forexample a twenty foot average of road conditions. When the frictionmonitor is used in conjunction with aircraft, the output block 113 couldbe coupled to a radio communication device 114 that directly transmitsrunway surface condition information to the control tower. In such acase, aircraft taxing down the runway or taking off or landing on therunway can transmit a detailed road surface condition (i.e., coefficientof friction condition) map of the runway condition along its route. Thisinformation can be very valuable to air traffic controllers incontrolling other aircraft in queue to land or take off.

[0050] The output indication could also be a driver prompt (visualand/or audible) 119 that indicates to the driver that the coefficient offriction of the road is high enough to be above the threshold of concernand this would prompt the driver to turn off the system and elevate thetest wheel 101.

[0051] Alternatively, or additionally according to the invention, theinformation on slipperiness can be provided to a vehicle control system140. The vehicle control system 140 may be a controller having amicroprocessor for combining the road surface information with data fromother vehicle systems 142 to facilitate safe vehicle operation. Forinstance, the vehicle systems 142 may be a speedometer or a steeringangle sensor, and the vehicle control system 140 could operate inco-operation with an anti-lock brake system 131 or generate warningsignals such as an unsafe stopping distance signal 132, an unsafecornering speed signal 133, etc. The skilled artisan will recognize thatthe vehicle systems 142 and safety features 131, 132, 133 are by way ofexample and not by way of limitation.

[0052]FIG. 1 shows that the electric motor 103 can also function as agenerator, and that a capacitor 135 can be attached to themotor/generator 103 to provide bursts of energy used to brake oraccelerate the test wheel 101. The electric motor/generator 103 may alsoprovide energy to the battery 137 so as to facilitate its operation.

[0053]FIG. 2 shows an embodiment of the invention in its most elementaryform, consisting of test wheel 101, test wheel rotation speed sensor105, a fixed (i.e., constant) electromagnetic or other resistance device102 and controller 109 a connected to output 113. In this “open loop”embodiment, there is a no feedback from the controller 109 a to alterthe strength of the resistance device 102. An on/off function isprovided, in that if the coefficient of friction on the ground issufficient to keep test wheel 101 rotating while fixed resistance device102 acts to resist this rotation, then one signal is provided to thedriver of the vehicle; conversely, if the conditions change and thecoefficient of friction beneath test wheel 101 is not sufficient tomaintain its grip on the wheel when resistance device 102 is active, andtherefore the test wheel 101 slips, then a different signal is providedto the driver. There are no means for variable resistance in thisembodiment. The driver is provided with a statement that a certain roadsurface friction threshold has been exceeded, but no quantificationbeyond this level is provided. Fixed resistance could be provided by anelectric motor or permanent magnets in place of the coils in theelectric motor. As described previously herein, a timer 123 withincontroller 109 a periodically initiates test cycles to providecontinuous monitoring of road surface conditions.

[0054] According to the invention, the fixed resistance of 102 can becombined with an alternator 202 to produce energy for the electricalsystem of this device, including a signalling system represented byoutput 113. A small battery 204 can also be incorporated to providepower when the generating function is limited, such as when there isvery slow movement of the vehicle. This self-powering, more elementarysystem of FIG. 2 is less expensive to produce and easier to install thanthe embodiment of FIG. 1. Moreover, the embodiment of FIG. 2 is moreeasily retrofitted to existing vehicles, aircrafts and the like.

[0055] This alternative embodiment of FIG. 2 is also simple andinexpensive in that it does not utilize the reference wheel tachometer111 of FIG. 1. One method to detect slip uses signals from the vehiclespeedometer 144 to the controller 109 a as a reference signal.Alternatively, slip is detected by the controller 109 a without such acomparison signal. The slip of the test wheel is detected by an analysisof the number of signals in any given period of time from the test wheelrotational speed sensor 105. FIG. 3 shows an embodiment of the testwheel rotation speed sensor 105. Pulses 307 are generated when markers303 on rotating shaft 301 pass in front of a scanner 305. For example,the test wheel rotation speed sensor 105 could be a magnetic or opticaldetecting device.

[0056]FIG. 3 also shows the motor 103 consisting of coils 310 andmagnets 320. The magnets are shown fixed to the rotating shaft 301 andthe coils 310 are fixed to the motor housing. A variable resistor isachieved by varying the voltage to the coils 310.

[0057] Optionally, as indicated above, the coils 310 may be replaced bypermanent magnets to provide a fixed resistance. Incidentally, when avariable resistance is desired, as in the embodiment of FIG. 1, thevariable resistance is achieved by varying the voltage to the coils 310as controlled by central processor 121.

[0058] In the embodiment of FIG. 2, the number of pulses 307 over agiven interval of time (the frequency) represent the rotational speed ofthe test wheel 101. When the test wheel is accelerated or braked, anincrease or decrease in pulses 307 in a given period of timedisproportionate to vehicle speed indicates that the test wheel 101 isslipping. For example, where the test wheel is braked, if a frequency ofpulses corresponding to a test wheel velocity of 25 mph suddenly changesto a frequency of pulses corresponding to 5 mph, and this change occursin a time period shorter than that needed for the vehicle to decelerateby this amount, then the test wheel 101 has slipped. A proportional,rather than absolute, decrease or increase (when accelerated) in testwheel velocity is used to detect slippage. For example, assumingconstant or near-constant (i.e., slowly changing) velocities, aone-third decrease or increase in the number of signals from the testwheel rotational speed sensor 105 in any given fraction of a secondcould be evidence of slip as distinct from deceleration or accelerationassociated with the more gradual slowing or speeding up of the vehicleitself.

[0059] In another open-loop embodiment of the invention, variablebraking resistance from the electric motor 103 is applied to the testwheel 101, although there is no feedback control to the electric motor103 from the controller 109 a. The applied variable braking resistancedoes not depend on the slip condition of the test wheel 101. Accordingto the invention, a plurality of predetermined and increasing brakingresistances are consecutively applied to the test wheel 101, and theslip condition is measured at each of the braking resistance levels. Forexample, if five braking levels are used, the test begins with theelectric motor 103 applying the first braking level and the controller109 a detecting whether the test wheel 101 slips; then the secondbraking level is applied and the presence of slip detected; then thethird, fourth, and fifth braking levels are applied in order, and thepresence of slip is detected for each braking level. Since the system isopen-looped, the five (for example) braking levels are appliedregardless of the outcome of the slip from a previous braking level.However, the driver is alerted to the existence of slip as soon as it isdetected through output 113 and indicators as shown in FIG. 1. Further,a quantification of the road surface friction may be displayed, ifdesired, by correlating the braking level at which slip first occurredwith the braking level for wheel lock which represents 100% slipcondition. This test method provides a finer quantification of roadsurface conditions than the fixed resistance method previously describedherein, while avoiding the complexity of a closed-loop feedback system.At each fixed level of resistance, the slip condition can be determinedfrom a sudden change in the number of pulses 307 inconsistent with avehicle deceleration condition. Similarly predetermined accelerativetorque is applied to the test wheel.

[0060]FIG. 4 shows a side view of the friction monitor. Connectingmember 402 is shown attached to rear axle 401 of an automobile. Member402 can be attached to any part of the vehicle, including the body.Preferably, member 402 is attached to the underside of the vehicle forease of operation. The test wheel 101 rotates in the same direction asthe vehicle wheels when the vehicle is moving forward. The test wheel101 can, if desired, be placed close to the track created by the wheelsof the vehicle. It is in contact with the ground while in operation andcan be lifted up by rotation about pivot 405 either manually ormechanically when not needed. A hook or latch is fixed to the vehicleframe and used to secure the wheel in a retracted (non-operational)position. FIG. 1 shows that an environment sensor 151 and automaticretracting means 153 (such as a motor) can be connected to thecontroller 109 to automatically elevate the test wheel 101. Theenvironment sensor 151 could be a moisture sensor, a temperature sensor,a device for measuring the reflectance of the road (used to detect ice),etc., and, when the sensor 151 indicates there is no need for roadsurface friction monitoring, the controller 109 automatically retractsthe test wheel 101.

[0061] Another feature according to the invention is a detachingcapability. The detaching means 407 allows member 403 to be instantlydetachable from member 402 if the test wheel 101 becomes caught on anobject during vehicle movement. This detaching capability may beprovided by, for instance, spring mounted restraints 409, similar tothose found in an umbrella telescoping support arm. The power lines forelectric motor 103 and signal lines from the controller 109 are coupledthrough a quick disconnect device which pulls apart when a “jerking”force above a certain threshold is applied.

[0062] Another feature to the invention is a folding mechanism thatoperates when fixed objects encounter the test wheel 101 from the rear.As shown in FIG. 5A, an obstacle 501 may encounter the test wheel 101from the rear and jam the test wheel assembly. This may occur, forinstance, when the vehicle moves in “reverse” and encounters a fixedobject. As shown in FIGS. 5A and 5B, the joints at pivot 405 and kneebend 413 allow the assembly to fold upwards and the vehicle to passsafely over the obstacle 501.

[0063] A mechanism is also provided as part of the suspension system toallow for movement of the test wheel 101, motor 103 and associatedcontrols in any direction so as to minimize the possibility of damage tothe measuring unit. As an example of such a suspension mechanism 601,FIG. 6 shows a plan view of one possible configuration of the frictionmonitoring device according to an embodiment of the invention. Mountingbracket 411 may be placed anywhere along the rear axle 401, such thatthe test wheel 101 may be located near the center of the vehicle or verynear to one of the vehicle tires. Note that the reference wheel 111 ismuch thinner than the test wheel 101, so that the majority of thevertical force on the test wheel assembly rests on the test wheel 101.Furthermore, the outer perimeter of the reference wheel 111 is made ofsoft material such that a greater percentage of the vertical force onthe assembly is brought to bear on the test wheel 101. Test wheel 101can be made in such a way as to minimize its mass in order to reduce itsmomentum and its effect on vehicle stability.

[0064] A constant vertical force of the test wheel 101 against theground is desirable to standardize measurements of the road surfacecondition, so that the same measured value means the same thing to thedriver of a truck as it does to the driver of a small car. This nearconstant vertical force is provided by the suspension means 415 andmembers 402 and 403 of FIG. 4. The suspension means 415 can be, forinstance, a deformable elastic material such as silicone, rubber, coil,spring, compressed air, etc., and is constructed so that the verticalforce of the test wheel 101 on the ground varies little with upward ordownward movement of the vehicle in relation to the ground. Thus, a nearuniform vertical force is maintained when the vehicle passes over bumpsor potholes in the road. The suspension means 415 is calibrated at thetime of installation of the test wheel 101, and further standardizationis achieved by making the test wheel width, surface contour, and surfacecoefficient of friction the same for all units. Furthermore, the wearcharacteristics of the test wheel 101 can be designed so that thesurface of the wheel has a constant coefficient of friction. Anadjusting means 417 to alter the spring-like resistance intermittentlyor continuously can also be incorporated. For example, if compressed airis used in the suspension means, the adjusting means 417 may increase(via a compressed air reservoir) or decrease the pressure therein.

[0065] Spring-like suspension between the surface of the test wheel andthe point at which the friction monitor is mounted on the vehicle canalso be achieved by making the connecting members 402, 403 between theelectric motor and the vehicle of flexible spring-like material.

[0066] According to an aspect of the invention, the vertical forcesensor 154 (FIG. 1), in the form of a strain gauge 419, can bejuxtaposed with the axle of test wheel 101 to continuously measure thevertical force on the test wheel 101 as shown in FIG. 4. Since thecoefficient of friction is a function of the vertical force on the testwheel 101 and the amount of torque required to brake or accelerate it,mathematical manipulation of the friction value quantification iscontinuously carried out to take into account the variations in thevertical force on the test wheel 101. By accounting for such variationsin the vertical force, the accuracy of the monitor is increased.

[0067]FIG. 7 shows an alternative physical configuration for thefriction monitor. The electric motor 103 is placed in the hub of thetest wheel 101 to facilitate operation under inclement conditions, suchas by preventing moisture from contacting the motor, etc. Note alsothat, according to the invention, the reference wheel 111 can be placedat any location along the axle 301.

[0068]FIGS. 8A and 8B show an alternative physical configuration for thefriction monitor. The electric motor 103 is placed closer to the pointof attachment of the monitor to the vehicle and works in co-operationwith the test wheel by means such as an enclosed belt drive 510.

[0069] The width of the surface of the test wheel can be as narrow as ablade. A narrower surface requires less torque to cause slippage andless vertical force to maintain representative contact with the surface.The test wheel diameter can be a mere fraction of what is shown.

[0070] In an alternative physical configuration for the frictionmonitor, the electric motor is placed closer to the point of attachmentof the monitor to the vehicle and works in co-operation with the testwheel by means such as an enclosed belt drive.

[0071]FIG. 9 shows an overall block diagram of the electromagneticsuspension and the friction monitor in which a vertical force electrictorque motor 901 is controlled by central processor 121. This embodimentis an improvement over the vertical force structures described inrelation to FIG. 4. In effect, in FIG. 9, the electric motor 901replaces element 415 of FIG. 4 to provide a more controllable verticalforce applying mechanism. Electric motor 901 can be used effectivelyeven without a vertical force sensor 154 since, like the slip torquemotor 103, the voltage or other electrical characteristic of motor 901can be used by the central processor 121 as a measure of the verticalforce and thus used in the determination of the calculation for the roadsurface condition. Of course, vertical force sensor 154 may also be usedin the embodiment of FIG. 9 for either calibration proposes or as anadded input for improved accuracy of the system. The motor 901 isconnected for applying the requisite vertical force as shown in greaterdetail in FIGS. 10 and 11.

[0072] While the term “vertical” force is generally used herein, it isunderstood that the coefficient of friction is calculated using theforce, or component of force, normal to the surface, so that a moreaccurate term is a normal force. In most applications, the normal forcewill be in the vertical direction where the surface, such as aroadway/runway, is horizontal.

[0073] In FIG. 9, the central processor quantifies the coefficient offriction utilizing signals representative of the vertical force appliedto the surface of the test wheel, and the torquing force required toproduce a given degree of slip of the test wheel. The central processorcan vary the vertical force applied to the test wheel and/or torquingforce to the test wheel (to produce slippage), in any given interval, inany sequence, and at any rotational speed of the test wheel, to maximizedata on road friction characteristics.

[0074]FIG. 10 illustrates the test wheel 101, with support member 1003attached to the axle of the test wheel 101 and pivotally attached to asupport member 1004. The casing of torque motor of 901 is attached to afixed point of support by member 1004 and the armature of the torquemotor is attached to support member 1003. Pivot 1001 can advantageouslybe placed at the same height above the measured surface as the axle oftest wheel 101. Support member 1004 may be pivotally attached to itspoint of attachment with electromagnetic solenoid attachment anddetachment means, to facilitate release. Test wheel 101 can be elevatedby reversing polarity to torque motor 101.

[0075]FIG. 11 shows the torque motor 901 with its casing attached tosupport member 1004 and armature 1106 attached to or integral withsupport member 1003.

[0076] While the invention has been described in terms of theembodiments illustrated, it will be appreciate by one of skill in theart that various modifications and improvements may be made within thespirit of the invention as defined by the appended claims. For example,while an electric torque motor is advantageously deployed to produce thevertical force, it will be appreciated by one of skill in the art,taking into account known methods of creating electromagnetic forcefields, that a solenoid may alternatively be used instead of the torquemotor, especially where applications require only small displacements ofthe test wheel.

What is claimed is:
 1. An apparatus for use with a vehicle forrepeatedly measuring road surface conditions, comprising: a test wheel,separate from the vehicle wheels, attached to said vehicle andpositioned for contact with a road surface, so as to rotate due tocontact with said road surface upon movement of said vehicle; means forapplying a variable electro-magnetic field torque opposing rotation ofsaid test wheel, so as to produce slippage of said test wheel; means formeasuring an amount of voltage utilized in said electromagnetic fieldapplying means and for providing a voltage measurement signalcorresponding thereto; means for measuring a rotation speed of said testwheel and generating a test wheel rotation speed signal; a controllerconnected to receive said test wheel rotation speed signal and saidvoltage measurement signal, said controller generating an output signalin response to a slip condition of said test wheel based on said testwheel rotation speed signal and said voltage measurement signal, andmeans for providing an indication of said output signal.
 2. Theapparatus as claimed in claim 1, further comprising: means forgenerating a reference signal indicative of the speed of said vehicle,wherein said controller is connected to receive said reference signaland generates said output signal in response to said test wheel rotationspeed signal and said reference signal.
 3. The apparatus as claimed inclaim 1, wherein said variable torque applying means is connected toreceive said output signal and varies the magnitude of said variabletorque in response thereto.
 4. An apparatus, for use with a vehiclehaving wheels, for repeatedly measuring road surface conditions,comprising: a test wheel, separate from the vehicle wheels, attached tothe vehicle and positioned for contact with a road surface so as torotate upon movement of the vehicle; means for applying a variable,electro-magnetic field, accelerative torque to produce slippage of saidtest wheel; means for measuring an amount of voltage utilized inapplying said accelerative torque and for providing a voltagemeasurement signal. corresponding thereto; means for measuring arotation speed of said test wheel and generating a test wheel rotationspeed signal; a controller connected to receive said test wheel rotationspeed signal and said voltage measurement signal, said controllergenerating an output signal in response to a slip condition of said testwheel based on said test wheel rotation speed signal and said voltagemeasurement signal; and means for providing an indication of said outputsignal.
 5. The apparatus as claimed in claim 4, further comprising:means for continuously producing a vertical force test signalrepresentative of the vertical force acting on said test wheel, andwherein said controller is further responsive to said vertical forcetest signal to produce said output signal.
 6. An apparatus, for use witha vehicle having wheels, for repeatedly measuring road surfaceconditions, comprising: a test wheel, separate from the vehicle wheels,attached to the vehicle and positioned for contact with a road surfaceso as to rotate upon movement of the vehicle; means for applying avariable torque to produce slippage of said test wheel; means formeasuring a rotation speed of said test wheel and generating a testwheel rotation speed signal; a controller connected to receive said testwheel rotation speed signal, said controller generating an output signalin response to a slip condition of said test wheel; and means forproviding an indication of said output signal.
 7. A method forcontinuously monitoring road surface friction, comprising the followingsteps: (a) positioning a test wheel, attached to a vehicle, for contactwith a road surface so as to rotate freely due to movement of saidvehicle along said road surface; (b) applying an increasing resistanceto said test wheel to resist rotation thereof; (c) detecting a firstinstance of slip condition of said test wheel on said road surface; (d)generating a signal representative of the braking torque required toproduce said incipient slip condition; and (e) providing an indicationof said slip detection signal.
 8. A method for continuously monitoringroad surface friction, comprising the following steps: (a) positioning atest wheel, attached to a vehicle, for contact with a road surface so asto rotate freely due to movement of said vehicle along said roadsurface; (b) applying an accelerative force to said test wheel toproduce slippage thereof; (c) detecting a first instance of said slipcondition of said test wheel on said road surface; (d) generating a slipdetection signal at the time of said first instance of a slip condition,said slip detection signal representative of a value of said torque atthe first instance of a slip condition; and (e) providing an indicationof said slip detection signal.
 9. A method for continuously monitoringroad surface friction, comprising the following steps: (a) positioning atest wheel, attached to a vehicle, for contact with a road surface so asto rotate freely due to movement of said vehicle along said roadsurface; (b) applying a variable torque to said test wheel to produceslippage therefor; (c) detecting a first instance of slip condition ofsaid test wheel on said road surface; (d) generating a slip detectionsignal at the time of said first instance of a slip condition; and (e)providing an indication of said slip detection signal.
 10. A method forrepeatedly testing road surface conditions, comprising the followingsteps: (a) positioning a test wheel, attached to a vehicle, for contactwith a road surface so as to rotate due to movement of said vehiclealong said road surface; (b) applying a fixed resistance to said testwheel to resist rotation thereof; (c) generating a slip detection signalindicative of a slip condition of said test wheel on said road surface;and (d) providing an indication of said signal.
 11. A method forrepeatedly testing road surface condition, comprising the followingsteps: (a) positioning a test wheel, attached to a vehicle, for contactwith a road surface so as to rotate due to movement of said vehiclealong said road surface; (b) applying a plurality of increasing,pre-determined resistances to said test wheel to resist rotationthereof; (c) generating a slip detection signal indicative of a slipcondition of said test wheel on said road surface for each of saidplurality of pre-determined resistances; and (d) providing an indicationof said signal for each of the said plurality of predeterminedresistances.
 12. A method for continuously monitoring road surfacefriction, comprising the following steps: (a) positioning a test wheel,attached to a vehicle, for contact with a road surface so as to rotatefreely due to movement of said vehicle along said road surface; (b)applying an increasing resistance to said test wheel to resist rotationthereof; (c) applying an electromagnetically controllable normal forceto bias said test wheel against said road surface; (d) detecting a firstinstance of slip condition of said test wheel on said road surface; (e)generating a signal representative of the braking torque required toproduce said incipient slip condition; and (f) providing an indicationof said slip detection signal.
 13. A method for continuously monitoringroad surface friction, comprising the following steps: (a) positioning atest wheel, attached to a vehicle, for contact with a road surface so asto rotate freely due to movement of said vehicle along said roadsurface; (b) applying an accelerative force to said test wheel toproduce slippage thereof; (c) applying an electromagneticallycontrollable normal force to bias said test wheel against said roadsurface; (d) detecting a first instance of said slip condition of saidtest wheel on said road surface; (e) generating a slip detection signalat the time of said first instance of a slip condition, said slipdetection signal representative of a value of said torque at the firstinstance of a slip condition; and (f) providing an indication of saidslip detection signal.
 14. A method for continuously monitoring roadsurface friction, comprising the following steps: (a) positioning a testwheel, attached to a vehicle, for contact with a road surface so as torotate freely due to movement of said vehicle along said road surface;(b) applying a variable torque to said test wheel to produce slippagetherefor; (c) applying an electromagnetically controllable normal forceto bias said test wheel against said road surface; (d) detecting a firstinstance of slip condition of said test wheel on said road surface; (e)generating a slip detection signal at the time of said first instance ofa slip condition; and (f) providing an indication of said slip detectionsignal.
 15. A method for repeatedly testing road surface conditions,comprising the following steps: (a) positioning a test wheel, attachedto a vehicle, for contact with a road surface so as to rotate due tomovement of said vehicle along said road surface; (b) applying a fixedresistance to said test wheel to resist rotation thereof; (c) applyingan electromagnetically controllable normal force to bias said test wheelagainst said road surface; (d) generating a slip detection signalindicative of a slip condition of said test wheel on said road surface;and (e) providing an indication of said signal.
 16. A method forrepeatedly testing road surface condition, comprising the followingsteps: (a) positioning a test wheel, attached to a vehicle, for contactwith a road surface so as to rotate due to movement of said vehiclealong said road surface; (b) applying a plurality of increasing,pre-determined resistances to said test wheel to resist rotationthereof; (c) applying an electromagnetically controllable normal forceto bias said test wheel against said road surface; (d) generating a slipdetection signal indicative of a slip condition of said test wheel onsaid road surface for each of said plurality of pre-determinedresistances; and (e) providing an indication of said signal for each ofthe said plurality of predetermined resistances.
 17. An apparatus forrepeatedly measuring road surface conditions, comprising: a test wheel,attached to a vehicle and positioned for contact with a road surface, soas to rotate due to contact with said road surface upon movement of saidvehicle; means for applying a variable resistance opposing rotation ofsaid test wheel; electromagnetic means for applying a controlled forceto said test wheel, said force normal to said surface; means formeasuring a rotation speed of said test wheel and generating a testwheel rotation speed signal; a controller connected to receive said testwheel rotation speed signal, said controller generating an output signalin response to a slip condition of said test wheel; and means forproviding an indication of said output signal.
 18. An apparatus forrepeatedly measuring road surface conditions, comprising: a test wheel,attached to a vehicle and positioned for contact with a road surface, soas to rotate due to contact with said road surface upon movement of saidvehicle; means for applying a variable accelerative torque to said testwheel; electromagnetic means for applying a controlled force to saidtest wheel, said force normal to said surface; means for measuring arotation speed of said test wheel and generating a test wheel rotationspeed signal; a controller connected to receive said test wheel rotationspeed signal, said controller generating an output signal in response toa slip condition of said test wheel; and means for providing anindication of said output signal.
 19. Apparatus as recited in claim 18,further comprising means for continuously producing a normal force testsignal representative of the normal force acting on said test wheel, andwherein said controller is further responsive to said normal force testsignal to produce said output signal.
 20. An apparatus for repeatedlymeasuring road surface conditions, comprising: a test wheel, attached toa vehicle and positioned for contact with a road surface, so as torotate due to contact with said road surface upon movement of saidvehicle; means for applying a variable torque to produce slippage ofsaid wheel; electromagnetic means for applying a controlled force tosaid test wheel, said force normal to said surface; means for measuringa rotation speed of said test wheel and generating a test wheel rotationspeed signal; a controller connected to receive said test wheel rotationspeed signal, said controller generating an output signal in response toa slip condition of said test wheel; and means for providing anindication of said output signal.
 21. The apparatus as claimed in claim17, further comprising: means for generating a reference signalindicative of the speed of said vehicle, wherein said controller isconnected to receive said reference signal and generates said outputsignal in response to said test wheel rotation speed signal and saidreference signal.
 22. The apparatus as claimed in claim 17, wherein saidvariable resistance applying means is connected to receive said outputsignal and varies the magnitude of said variable force in responsethereto.
 23. A device for applying a vertical force to a surfacefriction measuring test wheel comprising: (a) a test wheel moveableattached to a fixed point of support, a slip condition of said testwheel providing an indication of surface friction of a surface; and (b)electromagnetic force field applying means to bias said test wheel intocontact with measured surface.
 24. A method for applying a verticalforce to a surface friction measuring test wheel to measure a frictioncondition of said surface comprising the steps of: (a) providing a testwheel pivotally attached to fixed point of support; and (b)electromagnetically controllably biasing said test wheel into engagementwith said surface to produce a controllable contact force between saidtest wheel and said surface.
 25. A method for continuously monitoring asurface friction, comprising the following steps: (a) positioning a testwheel for contact with said surface so as to rotate freely due torelative movement of said wheel along said surface; (b) applying anincreasing resistance to said test wheel to resist rotation thereof; (c)applying an electromagnetically controllable normal force to bias saidtest wheel against said surface; (d) detecting a first instance of slipcondition of said test wheel on said surface; (e) generating a signalrepresentative of the braking torque required to produce said incipientslip condition; and (f) providing an indication of said slip detectionsignal.